Field of the Invention
The invention relates to a cryostat, in particular for use in a magnetic resonance imaging (MRI) system. Furthermore, the invention relates to a method for reducing heat input into a cryostat.
Description of the Prior Art
Superconducting magnet systems are used for medical diagnosis, for example in magnetic resonance imaging systems. A requirement of an MRI magnet is that it produces a stable, homogeneous, magnetic field. In order to achieve the required stability, it is common to use a superconducting magnet system which operates at very low temperature. The temperature is typically maintained by cooling the superconductor by immersion in a low temperature cryogenic fluid, also known as a cryogen, such as liquid helium.
The superconducting magnet system typically comprises a set of superconductor windings for producing a magnetic field, the windings being immersed in a cryogenic fluid to keep the windings at or below the superconducting temperature, the superconductor windings and the cryogen being contained within a cryogen vessel. The cryogen vessel is typically surrounded by one or more thermal shields, and a vacuum jacket completely enclosing the shield(s) and the cryogen vessel.
An access neck typically passes through the vacuum jacket from the exterior, into the cryogen vessel. Such access neck is used for filling the cryogen vessel with cryogenic fluids and for passing services into the cryogen vessel to ensure correct operation of the magnet system.
Cryogenic fluids, and particularly helium, are expensive and it is desirable that the magnet system should be designed and operated in a manner to reduce to a minimum the amount of cryogen consumed. Heat leaks into the cryogen vessel will evaporate the cryogen, which might then be lost from the magnet system as boil-off. In order to reduce the heat leaking into the cryogen vessel, and thus the loss of liquid, it is common practice to use a refrigerator to cool the thermal shields to a low temperature.
It is desirable that such superconducting magnet systems should be transported from the manufacturing site to the operational site containing the cryogen, so that they can be made operational as quickly as possible. In the case when the cryogen has been depleted, the system begins to warm-up and, if it exceeds a critical temperature, the magnet has to be pre-cooled with liquid nitrogen and then re-filled with the cryogen which is a time consuming and expensive process.
During transportation of an already assembled system, the refrigerator provided to cool the one or more shields and/or the cryogen vessel is inactive, and is incapable of removing the heat load from the shield and/or the cryogen vessel. Indeed, the refrigerator itself provides a low thermal resistance path for ambient heat to reach the cryogen vessel and shield(s). This in turn means a relatively high level of heat input during transportation, leading to loss of cryogen liquid by boil-off to the atmosphere.
It is desirable to reduce the loss of cryogen to the minimum possible, both since cryogens are costly and in order to prolong the time available for delivery, also known as the hold time, the time during which the system can remain with the refrigerator inoperable, but still contain some cryogen.
It is well known that the cold gas from evaporating cryogenic fluids can be employed to reduce heat input to cryogen vessels, by using the cooling power of the gas to cool the access neck of the cryogen vessel and to provide cooling to thermal shields by heat exchange with the cold exhausting gas.
Further, it has been demonstrated that removing the refrigerator and replacing the refrigerator with a heat exchanger can noticeably reduce the heat load onto the internal parts of the system, and therefore reduce the loss of cryogen. However, further improvement is desired.